Reactivating predcoef option.

[fedepedron]

This option was used when performing MD simualtions with AMBER (I suppose it is expandable to other packages as well).

The idea is to store several densities from previous MD steps and extrapolate to the current MD step, thus accelerating convergence. This is particularly useful for large systems (Heme-Fe, I'm looking at you) and implementing with the existing restart subs should be easy.